(a nephrotoxin) over a narrower part of its growth
range. Penicillium expansum, which causes soft rot of
apples, produces patulin (which causes hemorrhage of
the lungs and brains of experimental animals) over a
much narrower range.Physiological adaptations to water stressFungi typically respond to low (negative) external
water potentials by generating an even lower internal
osmotic potential, so that the cells remain turgid.
Sometimes this is achieved by selective uptake and
accumulation of ions from the environment, an
example being the common accumulation of K+by
marine fungi. However, high ionic levels are potentially
damaging to cells, and even the marine fungi seem to
take up K+primarily as a means of preventing the more
toxic Na+ion from entering the cell. A more common
method of balancing a high external osmotic envir-
onment is to accumulate sugars or sugar derivatives
that do not interfere with the central metabolic path-
ways. These osmotically active compounds are termed
compatible solutes. Glycerol is the most common
compatible solute in the highly xerophilic (drought-
loving) yeasts and filamentous fungi (Hocking 1993).
Comparisons between water-stress-tolerant fungi
and stress-intolerant fungi have shown that both
types produce compatible solutes in response to water
stress, but differ in their ability to retain the solutes.
For example, glycerol is the compatible solute of
both Saccharomyces cerevisiae(stress-intolerant) and
Zygosaccharomyces rouxii (stress-tolerant), and both
fungi produce it to the same degree when subjected to
water stress. But glycerol leaks from S. cerevisiaeinto
the culture medium whereas Z. rouxiiretains glycerol.
This has also been found in a comparison of the stress-
tolerant fungus Penicillium janczewskiiand the stress-
intolerant species P. digitatum. Membrane fluidity
thus seems to be implicated, and there is evidence of
a higher content of saturated lipids in the membranes
of water-stress-tolerant yeasts.
In recent years, increasing attention has focused on
the compatible solutes in fungal spores, and particu-
larly the spores of insect-pathogenic fungi. These fungi
have the potential to be developed as commercial
biological control agentsof insects, in place of some
of the toxic insecticides currently used (Chapter 15).
One of the main limitations at present is that the spores
need a sustained high humidity in order to germinate
and penetrate an insect cuticle. There is evidence that
spores with a high solute content can germinate faster
and at somewhat lower humidities than do spores with
low solute contents.
With this in mind, attempts are being made to
increase the levels of compatible solutes in spores of
insect-pathogenic fungi. These solutes can either be
derived from nutrient-storage reserves or from nutrients
taken up by the cells. Fig. 8.12 illustrates this for
two insect-pathogenic fungi, Beauveria bassianaand
Metarhizium anisopliae, grown on media adjusted to
different levels of osmotic stresswith either glucose
or trehalose. The data are difficult to interpret, because
many of these solutes are interconvertible, as we
saw earlier for the mycelia of the dry-rot fungus
(see Fig. 7.7). In the case of Beauveria, the compatible
solute content in the fungal spores increased markedly
as the solute concentration of the medium was
increased, but mannitol was the main compatible
solute in the spores when the fungus was grown on
glucose, whereas trehalose accumulated in the spores
when the fungus was grown on trehalose. A different154 CHAPTER 8
Fig. 8.11Combinations of temperature and water potential that support mycotoxin production by Aspergillus flavus,
Penicillium expansum, and P. verrucosum.(From Northolt & Bullerman 1982.)